Interpreting WAC number
 

I need help in reading WAC receipt number.
I have received two differnt I-797 - Payment of Notice - where the number is WAC119034xxxx and WAC19036xxxx.
Can someone help me to read this.
For all I am aware todate is that WAC = California Service Center; 11= the year; next 3 digits working days and last 5 digits are number of case received always starting with 5.
On my I797, what does 903 and 5 digits after that starts mean?
Does anyone know if there are changes to the original method of assigning case numbers? Thanks. Ash